1. Field of the Invention
This invention relates to metal detectors, and, more particularly, to metal detectors having the capability of reducing ground effects while discriminating between different types of metallic objects.
2. Description of the Prior Art
U.S. Pat. No. 4,128,803 uses a metal detector which has a transmit coil inductively coupled to a receiving coil and an oscillator connected to the transmit coil. The signals detected by the receive coil are processed by three synchronous demodulators, the first and second synchronous demodulators are used for detecting "R" or eddy current signal components and "X" or reactive signal components, respectively, and which two signal components are then used as inputs to the third synchronous demodulator. The output from the third synchronous demodulator is representative of the presence of a metallic object, and the polarity of the output signal indicates the particular type of the metallic object detected.
U.S. Pat. No. 4,024,468 discriminates between different types of metallic objects by amplitude discrimination. The amplitude of the received signal is representative of the type of metallic object detected. A tuning control is used to adjust the amplitude of a signal received from mineral soil to substantially eliminate the effects of ferrous mineral soils.
U.S. Pat. No. 4,030,026 discloses metal detector apparatus which uses a sampling technique in which the received signal is sampled and the sample voltage is utilized to produce an output signal corresponding to a selected component of the received signal. Since the output is based on a predetermined component of the received signal, the reactive components in the received signal due to mineral soils or other background conditions are simply ignored.
The dominant type of metal detector in contemporary use is a transmit-receive detector operating in the very low frequency portion of the radio frequency spectrum. This type of detector is usually referred to as a VLF/TR detector. The detectors are generally operated by moving a search head over the ground to be searched. The search head usually contains at least a single conductive coil which is coupled to an oscillator A magnetic field is generated by the transmit coil, and the generated field provides a sinusoidal wave corresponding to the frequency of the oscillator. Any metallic object passing into the magnetic field causes a reaction. The reaction, as received by a receive coil in the metal detector head, is ultimately presented to the user of the apparatus by some type of indicating element, or elements, such as a meter, an audible signal, etc.
In addition to discrete metal elements in the soil, which cause reactions from a metal detector, mineralization in the ground, usually ferrous oxides, also causes a reaction by metal detector apparatus, and the reaction is different from the reaction caused by conductive metals. Early metal detectors generally had two operating modes, a first mode for detecting conductive metals, and a second mode for detecting mineralized ground.
There is about a ninety degree difference between signals caused by soil mineralization and signals caused by conductive metals. By sampling the received signals ninety degrees out of phase with the oscillator allows minerals to be detected. A peak mineral signal usually occurs where the oscillator is crossing zero, which is a phase shift of about ninety degrees. This will be discussed in detail below, in conjunction with FIG. 1. For purposes of the present apparatus, and as is usual in the art, the sampling of the mineral signal at the time the oscillator output is zero, which is the time of maximum amplitude of the mineral soil signal, will be referred to as the mineral sample phase. This is generally referred to as the "reactive" or "X" component of the received signal.
The sampling of the received signal ninety degrees later, when the amplitude of the mineral signal is zero, is referred to as the metal sample phase, or the "resistive", "eddy current" or "R" component of the received signal. As a practical matter, the metal sample phase is usually sampled slightly more than ninety degrees after the mineral sample phase. However, for purposes of the present application, and as disclosed in the drawings to be discussed hereafter, the mineral sample phase and the metal sample phase or the "X" and "R" components of the received signal will be considered as ninety degrees apart. The practical metal sample phase, sampling as shown in FIG. 1, provides a stronger positive response to objects, such as coins, which are of prime concern to metal detector users. However, this also provides a stronger negative response for mineral signals, which is undesirable, than if the metal sample phase is sampled ninety degrees after the mineral sample phase, which is the time of the zero mineral signal.
The mass and surface area of a metal element affects the signal or the signature of the metal as detected with the metal detector. Once mineral soil is detected, the various signatures of the metals are relative to each other and generally fall out in the order given herein, as discussed below. The characteristics of the oscillator, input circuitry, and coil configuration used by a particular metal detector with respect to mineral soil is of primary importance and accordingly must first be determined. After the mineral soil is determined with respect to its signature and with respect to a particular metal detector, the other elements, such as discussed below, are generally fixed in order. For purposes of the present application, various elements are illustrated and are discussed, and their respective signatures are illustrated in wave forms and in vector diagrams. The order in which objects appear in phase with respect to each other is due to their metallic content and to the makeup of the objects and will generally be the same in all transmit/receive metal detectors.
Since the early detectors used a practical metal sample phase that allowed a large negative mineral response, the detectors were difficult to use over mineralized ground. When used in the metal mode, they tended to find all matter of metallic junk elements, such as nails, tinfoil, hair pins, bottle caps, etc. These two drawbacks were first solved on an individual basis.
The manufacturers of metal detectors discovered that by rotating the sample axis they could not only select whether the detector would respond to mineral or metal, but they could also discriminate between various commonly found metallic objects. According to the rotation of the sampling axis, various metallic elements may have either a positive component or a negative component.
By configuring the detectors' electronic circuitry to respond with an audio output or a meter indication for positive "R" axis signals, and no output or indication for negative "R" axis signals, a metal detector can be made to be nonresponsive to much of the unwanted and worthless metallic trash commonly found where people have congregated.
The techniques of discrimination discussed above are well known and are practiced by virtually all metal detectors in various configurations.
The type of signal demodulators used in metal detectors may be either synchronous or asynchronous, since the received signal in a transmit/receive detector is generated by the oscillator and modified in phase by the detected object. The receive signal is accordingly synchronized to the oscillator.
The '468 patent discussed above is typical of the type using an asynchronous demodulator. The demodulator used is a peak detector which responds positively to the decrease in amplitude of the received signals. The operating phase of the peak detector is established by biasing the receive coil signal with a phase shifted signal from the oscillator. The phase of the residual signal is set such that the addition of signals from desired objects causes a decrease in amplitude, and signals from unwanted objects cause an increase in amplitude.
Most metal detector apparatus in contemporary use have synchronous demodulators to provide the ability to discriminate. A phase variable signal is generated from the oscillated signal, and the phase variable signal is used as a reference signal for the synchronous demodulator. Varying the phase of the reference signal changes the sampling axis to provide the ability to discriminate, as discussed above, and as will be discussed in detail below.
The '026 patent discussed above is typical of current detectors using synchronous demodulators. The '026 apparatus covers the sampling of the received signal where the mineral signal is crossing zero, which is discussed above, and which is shown in FIG. 1 as the metal sample phase. This sampling point is indicated by the "R" vertical line in FIG. 1 and by the "R" axis in FIG. 2.
Sampling where the mineral components is zero frees the detector from adverse detuning effects of mineralized ground. By rotating the sampling axis, a metal detector may also be made to discriminate between various metal objects, as discussed more in detail herein. The '026 apparatus samples the received wave form at the mineral sampling zero crossover which frees the apparatus of mineral effects, but the apparatus is incapable of discriminating at that time.
The sampling axis, or "R" axis, is taken ninety degrees out of phase with the mineral signal, or "X" axis. Thus, the mineral signals will have no effect on the "R" sampling axis, but all of the metallic objects which are of common interest will have a positive component along the "R" sampling axis, as shown in FIG. 1.
By rotating the sampling axis more than 90.degree. with respect to the "X" axis, discrimination between the various metallic objects can be accomplished. However, the mineral component signal will have a negative component on the rotated sampling axis. Therefore, rotating the sampling axis could either provide mineral-free detection or discrimination between various metallic objects, but not simultaneously.
The general concept of the mineral-free sampling has been known for many years. It was first developed for early military mine detectors. Those early mine detectors used a pair of synchronous demodulators to detect the "X" and "R" components. However, the technique of using the two synchronous demodulators and other prior art techniques, were not able to provide mineral-free operation while discriminating between various types of metals. In the prior art, several methods have been devised to provide such discrimination and mineral-free operation, but they have not been without undesirable effects.
Since the received signal is a composite of a response of the magnetic field to all objects that have an effect on the receive coil, a readily apparent way to differentiate the various components of the composite wave form is based on the motion of the search coil over the ground. If the search coil were to be swept back and forth at a constant height above the ground, the tuning could be adjusted to cancel the mineral component of the wave form and to reestablish the operating point of the detector to correspond to the origin of vector diagrams such as included in FIGS. 2-6 herein. However, most operators tend to swing the search coil with a pendulum effect, since the pivot point of the detector is the operator's hand which is several feet above the ground. The pendulum effect means that there is a slow, rhythmic mineral signal that increases in amplitude as the coil is raised at each end of the swing. If the contour of the ground changes, there will be faster mineral changes with respect to the amplitude of the signal. However, this effect is relatively slow.
As the search coil passes over a metal object, such as a coin, the response is very rapid due to the brief time duration that the search coil is disposed over the metal object. Most attempts to provide mineral-free operation and to simultaneously discriminate have used this frequency domain difference to provide the desired results.
One way to solve the discrimination problem and mineral-free operation at the same time is by way of feedback circuitry incorporating time delays in the feedback loops. The feedback response is intended to be fast enough to cancel the slow mineral signal, and yet at the same time be slow enough to react to the higher frequency components of the metallic target response. However, this technique has an undesirable effect in that to be of substantive value in helping tune out the mineral effects, the feedback must be reasonably fast. When a metallic target is passed over, the feedback compensates for large portions of the metallic target response, which weakens the response to the object itself. At the same time, when the search coil passes over an object, the opposing feedback signal tends to bias the receive coil in the opposite polarity until the time delay allows the demodulator to sense the result and to remove the opposing feedback signal.
To illustrate the statements contained in the preceding paragraph, an example may be appropriate. As a detector coil is passed over a nail buried in mineral soil, a wave form without feedback would indicate a negative response, assuming the "R" sample axis is set at about 225.degree., which would theoretically eliminate or discriminate between a coin and unwanted or undesirable metallic trash objects. With a delayed feedback, the effect of the nail is substantially cancelled, but then a signal is generated which corresponds to a 180.degree. phase reversal as the nail effect is lost but the feedback effect is still present. Thus, the opposing wave form generated due to the feedback effect indicates that the nail is actually a good or desirable object.
The feedback concept still results in problems which are undesirable. Other attempts have been made to solve the undesirable problems, such as a.c. couplings or low pass filtering. These also have resulted in the same undesirable effects.
In the '803 patent, briefly discussed above, a detector is described which overcomes the undesirable effects by using three synchronous demodulators and matched band pass filters. Two of the synchronous demodulators are used to detect the components of the composite signal where the mineral signal is zero and at some desired discriminate setting. The mineral-free demodulator is referred to as the "R" demodulator, and the discriminating demodulator is referred to as the "X" demodulator. The sample axis of the "X" demodulator may be rotated to provide the desired degree of discrimination, as discussed above.
In the '803 patent, the outputs of both the "X" and "R" demodulators are passed through band pass filters which remove any low frequency signal components and provide what is referred to as "ringing" signal outputs at a frequency of about twenty Hz. These signals both exhibit the previously discussed effects of having a large 180.degree. flyback signal as the result of their having lost their d.c. reference component in the filters. The purpose of the third synchronous demodulator is to provide d.c. restoration to the "X" signal, which substantially eliminates this undesirable effect. The "R" signal is used as a reference to demodulate the "X" signal. The inputs to the third synchronous demodulator are the two filtered "X" and "R" signals.
Since both filter signals are generated by the same metallic object, the "R" signal prior to filtering will be a positive pulse, and the "X" signal, prior to filtering, will be positive for a desired object and negative for an undesired object. Accordingly, the ringing signal outputs of the filters will be in phase if the pulses are both positive, or out of phase if the "X" signal is negative.
The third synchronous demodulator looks for positive "X" signals when "R" is positive. It also looks for negative "X" signals when the "R" signal is negative. When the "R" signal changes polarity due to the ringing of the filtered signals, the third synchronous demodulator has the effect of reversing the X demodulator sample axis. Depending on the polarity of the R signal, undesirable objects will provide one type of output signal, and desirable objects will provide a different type of signal.
The approach of the '803 patent, while providing a solution of the undesirable effects of a.c. coupling or filtering, also has some inherent undesirable effects. For example, the filters used must be very well matched in response since the elimination of the mineral ground effects is accomplished entirely by the filters and since the third synchronous demodulator is phase sensitive. Moreover, any phase delay or difference between the two ringing signals of ninety degrees or more over the duration of the ringing signals may be interpreted by the third synchronous demodulator as a 180.degree. phase reversal of one of the signals. This would appear as the same effect as a delayed feedback discussed above, and an undesirable object would accordingly cause both a negative response and a positive response when the phase difference exceeds ninety degrees.
Another problem with the apparatus of the '803 patent is cost. The matching of components, including the filters, resistors, and capacitors is relatively expensive, and the labor required to match the components is also relatively expensive.
The apparatus of the present invention overcomes the problems of the prior art, as discussed above, without requiring a third synchronous demodulator and the matched components needed in the '803 patent and overcomes the problems of the prior art as discussed in general above.